The invention relates to a receiving part for connecting a shank of a bone anchoring element to a rod and to a bone anchoring device with such a receiving part to be used in spinal surgery or trauma surgery.
A known polyaxial bone screw 100 is shown in FIG. 11. It comprises a receiving part 101 for connecting a screw element 102 to a rod 103, the outer wall of the receiving part being substantially cylindrical. The receiving part 101 has a recess 104 with a substantially U-shaped cross-section forming two open legs 105, 106 defining a channel for inserting the rod 103. An inner thread 107 is provided at said legs for receiving a securing screw 108 to fix said rod in the channel. The inner thread 107 is a metric thread.
When screwing in the securing screw 108 as shown in FIG. 13, forces directed radially outwards which are depicted by arrows A act on said inner thread which cause said legs 105, 106 to splay. This may loosen the fixation of the rod.
Several approaches have been made so far to solve the problem of the splaying of the legs. One solution is to provide tube-shaped counter-holding tools. Another solution consists in providing an outer nut to be screwed onto an outer thread of said legs as, for example, disclosed in EP 0 614 649 A. Also, outer ring-shaped securing means are known. However, securing means such as outer nuts or rings enlarge the size of the bone screw and therefore limit the clinical applications.
A further approach consists in using a specific kind of thread shape for the inner thread 107 and the corresponding thread of the securing screw. It is known to use a saw tooth thread as disclosed in U.S. Pat. No. 5,005,562 or a reverse angle thread such as disclosed in U.S. Pat. No. 6,296,642 B1 or US 2002/0138076 A1 to eliminate the outwardly directed radial forces acting on said legs. A particularly advantageous thread shape is the flat thread as disclosed in US 2003/0100896 A1 which eliminates such radial forces and is easy to manufacture. The use of these specific thread shapes allows a use of the bone screw without a further outer securing device such as an outer nut or ring.
However, despite the above measures there is still a problem of a diagonal splaying of the legs at the time of final tightening of the securing screw. This splaying is caused mainly by the torque acting on said legs 105, 106 at the time of final tightening which causes a diagonal lateral deformation of the open ends of the legs in the longitudinal direction of the rod due to the friction between the thread flanks. The torque B acting on the legs is shown in FIG. 2. The deformation is shown in FIG. 14 which does not show the securing screw for the purpose of better illustration. The problem of the splaying of the legs is not restricted to polyaxial bone screws but also occurs in all so called top open bone anchoring devices including monoaxial screws which have two open legs and a slit in between for inserting the rod.
Providing flattened surfaces 109 at the sides of the receiving part which are oriented in a transverse direction of the channel, as shown in FIG. 12, does not solve the problem but contributes to minimize the size of the bone screw in a longitudinal direction of the rod.